Literally 'messenger' — used for both divine angels and human messengers. In the NT, angelos primarily refers to supernatural beings who serve God, deliver messages, execute judgment, and minister to believers. The word gives us 'angel' in English.
Angels appear at every critical moment: announcing Christ's birth (Luke 2:10), strengthening Him in Gethsemane (Luke 22:43), announcing the resurrection (Matthew 28:5), and directing the early church (Acts 12:7). Hebrews 1 establishes Christ's supremacy over angels. The 'angels of the seven churches' in Revelation may be heavenly guardians or human leaders — the ambiguity of angelos is intentional.